Art of metalizing nonmetallic bodies



Dec. 14, 1948. v L. PESSEL 2,456,082

ART OF METALLIZING NONMETALLIC BODIES Filed Sept. 30, 1943 Snnentor LEOPIILD PEI/EL- attorney mama Dec. 14, 194a ART OF METALIZIN G NONMETALLIC BODIES Leopold Pessel,

oi Delaware Philadelphia, Pa., assignor to Radio Corporation of America,

a corporation Application September 30, 1943, Serial No. 504,495 4 Claims. (Cl. 117-35) My present invention relates to the art of depositing silver upon piezoelectric crystals, glass and other non-metallic bodies and has for its principal object to provide an extremely rapid, economical and trouble-free method for the mass production of metallized elements of the general character described.

While my invention will be described as applied to the formation of adherent metal electrodes, in a desired pattern, upon a number of piezoelectric crystal elements it will be apparent as the description proceeds that my invention is likewise applicable to the mass production of mirrors and other metallized objects.

It has previously been proposed to employ methods commonly used in the silverlng of mirrors for metallizing piezo-electric crystal elements. The methods most commonly employed in the deposition of metal on glass are the Brashear Formula method and the Rochelle Salt Formula method (see U. S. Bureau of Standards Circular No. 389). Irrespective of the advantages claimed for each formula it may be said, generally, that the Brashear Formula method is objectionable because of the explosive and caustic nature of the chemicals employed and that the Rochelle salt method, as heretofore employed in the plating of piezoelectric crystals, is most ineflicient, principally because of the difliculties incident to depositing the silver in suiiicient quantities to provide electrode coatings thick enough to permit the frequency of the crystals to be altered by burnishing the metal. In this latter connection it may be observed that the chemicals have a certain reaction time and that the plating operation must be completed within this period if numerous replating operations with their attendant decrease in silver yield and increase in plating time are to be avoided.

Considered from one aspect, my present invention constitutes an improvement upon the R- chelle Salt Formula method and may be said to reside not only in the novel chemical formulas which I have devised but also in the several novel mounting, masking, sensitizing and other mechanical and chemical features hereinafter described and which enable me (a) to reduce the plating time of piezoelectric crystals to about onefourth the time previously required and (b) to accomplish this saving with a silver consumption approximately one-seventh of that heretofore required to provide electrode coatings of comparable thickness and area.

In the accompanying drawings Figure 1 is a side elevational view partly in section of an apparatus which I may use in carrying my invention into effect and Figure 2 is a front elevation of a frame forming part of the apparatus of Figure 1.

Referring now to the drawings: In carrying my invention into eflect the crystals, which are designated I, are fastened in rows to the adhesive surfaces of parallel strips 3 of masking tape (e. g. "Scotch tape) in such a manner that the tape covers diametrically opposite end portions of the crystal elements. Suflicient pressure is applied to the crystals to ensure firm adhesion. The strip assemblies are fastened in spaced relation to frames 5, for example in the manner shown in Figure 2, so that the cleansing and plating solutions may be circulated therebetween with the crystals being dislodged when the frames are rocked within the plating tank I or subjected to relative motion of the plating solution with respect to the frames. With the crystals thus assembled the crystals are cleaned in an alkaline bath. I prefer to use a bath containing 2 per cent trisodium phosphate and 2 per cent of a wetting agent (such, for example, as a high molecular alcohol-ether condensate. e. g. "Triton 720 made by Rohm and Haas Company), although other suitable cleansing agents can be used. After cleansing and rinsing in water the crystal assemblies are sensitized in a sensitizing bath comprising a dilute stannous chloride solution (about 0.3 per cent), and containing about 2 per cent of a non-ionized wetting agent of the high molecular alcohol-ether type such, for example, as one of those more fully described and claimed in my copending application, Serial I Number 504,494, filed concurrently herewith and now abandoned. After additional rinsing, the crystal assemblies are washed in a hot water bath. The temperature of the water is preferably at about 150 F. although lower or higher temperatures may be used as long as they are substantially above room temperature The A" and "3 solutions of the Rochelle Salt Formula are held at a temperature at least slightly below room temperature (below F.)

preferably at about 65 I"., are mixed in the proper proportion, and the preheated crystalframe combination immersed into them. Controlled stirring is applied, either by mechanically rocking the frames in the liquid, or by passing the liquid past the stationary frames, or by a combination of motion of liquid and frames. After a certain time, that may vary between and 30 minutes, the frames are removed and, if necessary, inserting into a fresh plating solution where the procedure outlined above is repeated.

After the desired amount of silver is deposited on the frames, they are removed from the plating solution, rinsed in water, preferably in warm or hot water, and dried. The crystals are then removed from the tapes and are ready for further testing and finishing operations.

The silver plating solutions of the Rochelle Formula are customarily composed of "A" and B solutions, which are kept separately and mixed immediately preceding the plating operation. The A solution contains preferably 20 grams of silver nitrate, corresponding to about 12.7, grams of silver, per liter, in ammoniacal solution.

The "B" solution is customarily prepared by dissolving 4 grams of silver nitrate per liter and precipitating with 3.3 grams Rochelle salt (sodium potassium tartrate). After filtering and cooling, this solution constitutes a saturated solution of silver tartrate, containing silver in amounts varying between 1.0 and 1.5 grams per liter.

In the past, "A" and "8 solutions were mixed in volume proportions of 1:1. I have found, however, that much higher yields, and greatly improved plating efllciency can be obtained if these A:B ratios are substantially smaller than 1:1. I prefer to use an A:B ratio of 1:4, although higher or lower ratios may be used, as long as they are substantially smaller than 1:1. As stated in the preceding paragraphs, I may first make up the B solution as is customarily done. This solution will then contain from 1 to 1.5 grams per liter of silver in the form of the tartrate. In the past, since the A and B solutions were used in a 1:1 ratio and since the A" solution customarily contains about 12.7 grams per liter of silver in the form of an ammoniacal complex, the customary proportions by weight in the completely mixed silvering solution have been from 1-1.5 grams of silver as tartrate to 12.7 grams of silver as ammonia. But'I prefer to use four times as much of the 3" solution as formerly used; hence, my preferred solution will contain by weight 4 (1-1.5) or from 4-6 grams.

of silver as tartrate to 12.7 grams of silver as ammonia. Expressed as a decimal ratio of B/A, the lower limit of this range will then be 4/12.! or 0.315, which is about 0.3. Similarly calculated, the upper limit of my preferred range of B/A is equal to 6/12.! or 0.472, which is about 0.5. Therefore, my preferred range of B/A, i. e., silver present as tartrate to silver present as ammonia, may be stated to be from about 0.3 to about 0.5, since, as I have previously stated, the exact values which I have given as examples are not intended to be used without some deviation. The use of an increased volume of B solution, as compared with that formerly used in this process, is done in order to overcome the low solubility of silver tartrate, and make relatively larger roportions of this reducing agent available for the reduction of the ammoniacal silver solution.

The "3 solution may be supersaturated, as to tions may be prepared by boiling solid silver tartrate, with or without the addition of other salts, colloidal materials, etc. with water, until the desired amount of silver has gone into solution. I

have found that B" solutions showing these characteristics in a pronounced manner ar particularly effective in producing heavy deposits.

By combining the various features outlined above, I have been able to'reduce the plating time to about one-fourth of the time previously required and to accomplish this with a silver con sumption of one-seventh of the amount previously required. Due to the great increase of plating efficiency, the silver content of the residual waste solution can be decreased to a small fraction of that found previously in the waste solutions.

I claim:

1. A method of metallizing a non-metallic object which comprises dissolving 20 grams of silver nitrate in one liter of ammonia water to form an ammoniacal silver solution, mixing 4 grams of silver nitrate per liter with 3.3 grams of sodium potassium tartrate to form a saturated silver tartrate solution, mixing the two solutions in a volumetric proportion of from about 1 to about 4 whereby the resulting solution exhibits a ratio of silver present as tartrate to silver present as ammonia compound of the order of 0.3 to 0.5, and then immersing said object in said solution.

2. A method of metallizing a non-metallic body which comprises sensitizing the surfaces of the body to be metallized, washing said sensitized body and then immersing the said washed and sensitized body in a bath consisting of a mixture of two solutions A and B, of which solution A consists of ammonia water containing 20 grams of silver nitrate per liter of said ammonia water and solution B consists of 4 grams of silver nitrate and 3.3 grams of sodium potassium tartrate per liter of water, the resulting mixture being made such that it contains a ratio of silver present as tartrate to silver present as ammonia compound of the order of 0.3 to 0.5.

3. A method of metallizing a piezoelectric crystal which comprises immersing said crystal in a dilute stannous chloride solution containing a non-ionized wetting agent of the high molecular weight alcohol-ether type to sensitize the surfaces to be metallized, removing said crystal from solution and washing the same, immersing the sensitized and washed crystal in a bath consisting of a mixture of two solutions A and B, of which solution A consists of ammoniacal silver nitrate and solution B consists of silver tartrate, said mixture being made up such that it contains a ratio of silver present as tartrate to silver present as ammonia compound of the order of 4. A method of metallizing a non-metallic object which comprises immersing said object in a bath consisting of a mixture of two solutions A and B, of which solution A consists of ammoniacal silver nitrate and solution 3 consists of silver tartrate, said mixture being made up such that it contains a ratio of silver present as tartrate to silver present as ammonia compound of the order of 0.3 to 0.5.

moron) PESSEL.

REFERENCES CITED The Ioiiowin: references are of record in the me of this patent:

UNITED STATES PATENTS Name Number Date Poissonnier June 19, 1860 Number 

